If you call the microflow in which you call the REST service from a button, in the button properties you can change the settings for the microflow:
There you can set the microflow to blocking, which will show a progress bar. You can even add some text if you like:
Hope this answers your question!
Are you calling the REST service from a microflow button or from a scheduled event
If from a button, then you could set a progress bar in the microflow-button-settings
When you call your microflow, just add these settings as shown below , you will get progress bar with the message
“ Retrieving Data”